The present invention relates to a process for the recovery of uranium from wet-process phosphoric acid. More particularly, the present invention relates to a liquid-liquid extraction process for the recovery of uranium from wet-process phosphoric acid, using a high-efficiency extractant exhibiting a relatively high degree of thermal stability.
Wet process phosphoric acid, which is an intermediate in the production of fertilizer, is prepared by reacting sulfuric acid with phosphate rock (calcium phosphate). The reaction between sulfuric acid and phosphate rock produces a calcium sulfate hydrate and phosphoric acid. The calcium sulfate is separated from the reaction mixture to produce a phosphoric acid product suitable for several uses, including the production of phosphate components for fertilizers.
It is known that certain phosphate rock contains small quantities of uranium, and that the phosphoric acid prepared from the phosphate rock also contains small quantities of uranium. Although the concentration of uranium in this so-called "wet process phosphoric acid" is quite small, the amount of wet process phosphoric acid which is produced each year is so great as to constitute a potential source of significant amounts of uranium.
To this end, several methods have been developed for recovering uranium from wet process phosphoric acid.
In accordance with one method, phosphoric acid at about 30% concentration (as P.sub.2 O.sub.5) is "reduced" by the addition of metallic iron thereto. This reduces the iron present to the Fe.sup.+2 state, and the uranium to the U.sup.+4 state. The uranium is then extracted with a 5% solution of capryl pyrophosphoric acid in a kerosene type solvent, and the 30% phosphoric acid returned to its originally intended purpose (i.e., fertilizer manufacture or the like). The organic solvent is then treated with HF to precipitate-out the uranium as UF.sub.4.
One unfortunate characteristic of this process is, however, that capryl pyrophosphoric acid, besides having the hydrolytic instability characteristic of pyrophosphates generally, is also thermally unstable.
In accordance with a second method, the uranium in the wet process phosphoric acid is first oxidized, so that most or all of it is in the U.sup.+6 state. The uranium is then extracted with an organic (i.e., kerosene) solution of a mixture of di-(2-ethylhexyl) phosphoric acid and trioctyl phosphine oxide. The uranium is then stripped from the organic solution and further concentrated by contacting it with a smaller volume of 30% phosphoric acid containing a reducing agent, such as divalent iron. This cycle may be repeated several times, or practiced in a multi-stage counter current configuration. As a final step, the uranium is recovered from the organic solvent by contacting with an aqueous solution of ammonium carbonate, and precipitated therefrom as ammonium uranyl tricarbonate. The ammonium uranyl tricarbonate can then be calcined to form U.sub.3 O.sub.8.
Yet another method is taught by U.S. Pat. No. 3,835,214. In accordance with the teaching of that patent, uranium is extracted (in the U.sup.+4 state) from wet-process phosphoric acid with an organic (i.e., kerosene) solution of mono- and di-(octyl-phenyl esters of orthophosphoric acids wherein the octyl-phenyl group is specifically para(1,1,3,3 tetramethylbutyl). The uranium is then stripped from the organic phase by a phosphoric acid containing an oxidizing agent, such as Na.sub.2 S.sub.2 O.sub.8,Cl.sub.2,O.sub.2, ozone, H.sub.2 O.sub.2 and NaClO.sub.3. The uranium (now in the U.sup.+6 state) is then extracted from the strip solution with an organic solution of di(2-ethylhexyl) phosphoric acid and trioctylphosphine oxide. Finally, the uranium is recovered from the organic solution by contacting it with an ammonium carbonate solution, which converts the uranium to uranyl tricarbonate.
These three prior art methods have been employed, with varying degrees of success, in recovering uranium from wet process phosphoric acid produced by the "dihydrate" process (so named because it results in the formation of CaSO.sub.4.2H.sub.2 O). Such acids are generally characterized as having concentrations, calculated as P.sub.2 O.sub.5, of less than 40% by weight.
An improved method has recently been developed for the production of wet process phosphoric acid. This process, known as the "hemihydrate" process (because it results in the formation of CaSO.sub.4.O.5H.sub.2 O), forms wet process phosphoric acids having P.sub.2 O.sub.5 concentrations in excess of 40%. This method is regarded as an improvement over the dihydrate process because it has more favorably energy requirements, since the amount of water which must be removed to concentrate the acid is substantially reduced.
Unfortunately, however, the extraction of uranium from wet process phosphoric acid becomes more difficult as the concentration of the acid is increased, and the prior art uranium recovery methods have been less than satisfactory for recovering uranium from phosphoric acid produced by the hemihydrate process.
Therefore a need exists for a new method for recovering uranium from phosphoric acid.